PSI to PSIG Calculator Equation and Pressure Guide

To convert psi to psig, subtract the local atmospheric pressure from the absolute pressure: psig = psi − atmospheric pressure in psi. Enter absolute pressure and atmospheric pressure to get gauge pressure; at standard sea-level pressure, 14.696 psia converts to 0 psig.

Formula

Gauge pressure = absolute pressure − atmospheric pressure

In symbols: P_g = P_abs − P_atm, with all pressures expressed in the same units.

• P_g is gauge pressure, shown here in pounds per square inch gauge (psig).
• P_abs is absolute pressure, shown here in pounds per square inch absolute (psia).
• P_atm is the surrounding atmospheric pressure, also in psia.

The calculator defaults to 14.696 psia for both absolute pressure and atmospheric pressure, so its default result is 0. That atmospheric value is the conventional standard atmosphere at sea level; actual local pressure varies with elevation and weather. Use a measured or otherwise appropriate local atmospheric pressure when accuracy matters.

Worked Example

Using the calculator’s default values:

1. Absolute pressure: 14.696 psia.
2. Atmospheric pressure: 14.696 psia.
3. Subtract: 14.696 − 14.696 = 0 psig.

Rounded to the calculator’s two decimal places, the result is 0.00 psig. The result is zero because the entered absolute pressure equals the entered surrounding atmospheric pressure.

What PSI, PSIA, and PSIG Mean

“Psi” means pounds per square inch, a pressure unit. By itself, psi does not always identify the pressure reference. In everyday contexts, a reading described only as psi may be gauge pressure, but technical specifications should state whether pressure is absolute or gauge because the distinction changes the value.

• PSIA means pounds per square inch absolute. It is measured from a perfect vacuum, which is zero absolute pressure.
• PSIG means pounds per square inch gauge. It is measured relative to the surrounding atmospheric pressure. A gauge reads zero when its pressure is the same as the surrounding air pressure.
• Atmospheric pressure is the pressure of the surrounding air. It is not a fixed value everywhere or at all times.

Gauge pressure can be positive or negative. A negative psig reading means the pressure is below the surrounding atmosphere; it does not mean the absolute pressure is negative. Absolute pressure cannot be less than zero.

How to Convert PSI to PSIG

First determine what the given “psi” value represents. If it is already gauge pressure, it is already in psig and no conversion is needed. If it is absolute pressure, subtract the atmospheric pressure for the location and conditions relevant to the measurement.

For example, if a system is at 30.00 psia and the surrounding atmosphere is 14.70 psia, then 30.00 − 14.70 = 15.30 psig. If the system is at 12.00 psia under that same atmospheric pressure, the result is 12.00 − 14.70 = −2.70 psig. That negative gauge value means the system pressure is 2.70 psi below the surrounding atmosphere.

If the inputs are in different pressure units, convert them to a common unit before subtracting. For reference, 1 psi is approximately 6.89476 kilopascals (kPa), and 1 atmosphere is 101.325 kPa. The same relationship works in any consistent pressure unit: absolute pressure minus atmospheric pressure equals gauge pressure.

When to Use the Conversion

This conversion is useful when comparing an absolute-pressure measurement with a gauge reading, interpreting technical data, or converting pressure values between reference systems. It also explains why a pressure instrument open to the air reads zero gauge pressure even though the atmosphere itself has substantial absolute pressure.

Use the calculator only after identifying the reference for each number. A sensor or instrument specification may report absolute pressure, gauge pressure, or a differential pressure between two points. Differential pressure is a separate comparison and should not automatically be treated as atmospheric pressure. Check the instrument documentation or measurement context when the reference is unclear.

Common Errors and Limits

• Assuming every psi value is absolute. In many practical settings, an unlabeled pressure reading is gauge pressure. Confirm whether the source says psia or psig before calculating.
• Adding atmospheric pressure instead of subtracting. To get gauge pressure from absolute pressure, subtract the surrounding atmosphere. Adding it is the reverse conversion: psia = psig + atmospheric pressure.
• Using a fixed sea-level value for every location. 14.696 psia is a standard reference, not a universal local measurement. Atmospheric pressure changes with altitude and weather; use a suitable local value for a precise conversion.
• Mixing units or rounding too early. Both pressures must use the same unit. Keep sufficient precision through the subtraction and round the final result to a sensible number of decimal places.
• Interpreting negative gauge pressure as negative absolute pressure. Negative psig is possible when pressure is below ambient. Absolute pressure remains zero or greater.

The equation is a reference conversion, not a measurement correction for every instrument or process. It does not account for sensor calibration, elevation-specific equipment behavior, temperature effects, or a pressure difference between locations. For equipment selection or pressure limits, use the relevant technical specifications and qualified guidance rather than relying on this arithmetic alone.

Frequently Asked Questions

What is the equation to convert psi to psig?
When the input is absolute pressure, subtract atmospheric pressure: psig = psia − atmospheric pressure. The values must be in the same units.

Is psi the same as psig?
Not necessarily. Psi names the unit, while psig specifies that pressure is measured relative to the surrounding atmosphere. A psi value may be ambiguous unless its reference is identified.

How do I convert psia to psig at sea level?
Subtract the chosen sea-level atmospheric reference, commonly 14.696 psia. For example, 30.00 psia − 14.696 psia = 15.304 psig, or about 15.30 psig.

Can psig be negative?
Yes. A negative gauge value means the measured pressure is lower than the surrounding atmospheric pressure. It does not mean the absolute pressure is below zero.

How do I convert psig back to psia?
Add atmospheric pressure: psia = psig + atmospheric pressure. Use the atmospheric value relevant to the measurement, and keep all pressures in the same units.

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in a | Absolute pressure | psia | 14.696 | 0
in atm | Atmospheric pressure | psia | 14.696 | 0
out g | Gauge pressure | psig | a - atm | 2